Danish Patent No. 149238 discloses a zero-flux current transformer for measuring the current for an electromagnet in a particle accelerator. As very high current values are involved, viz. several hundred amperes, the main current is advantageously converted into a comparatively weaker measuring current which is easy to handle, and which is passed through a measuring resistor. As a result the voltage drop across the measuring resistor can be used as a measurement representing the value of the main current.
The known zero-flux current transformers are formed by a combination of a magnetic integrator and a 2nd harmonic modulator. In principle a magnetic integrator includes a ring core of a ferromagnetic material, said ring core being provided with a primary winding, a secondary winding and a sensing winding. The sensing winding communicates with the inlet terminals of an amplifier driving the secondary current through a measuring resistor. Therefore an alteration of the magnetic flux in the ring core induces a voltage in the sensing winding, and this voltage affects the amplifier in such a manner that said magnetic flux generates a compensating current which counteracts the alteration of the flux caused by the primary current. As a result the magnetomotive forces generated by the current through the primary winding are equalized by the magnetomotive forces generated by the current through the secondary winding in such a manner that a specific relationship exists between the current value in the primary winding and the current value in the secondary winding.
However, the magnetic integrator cannot handle DC and very low frequencies, and accordingly it is necessary to include a separate circuit for this function. Such a circuit is formed by a magnetic modulator including two identically wound ring cores and a driver circuit. The ring cores are driven in saturation, and at a balance/“zero flux” the current curves are symmetrical relative to zero with the result that the contents of equal harmonics are zero. Therefore it is possible to use either a direct, symmetrical detection by means of a window comparator/Schmitt trigger or a second harmonic detector using synchronous rectification of the current signal presenting the double frequency of the modulation signal. In both cases an output signal is obtained which is zero at a balancing between the primary and the secondary ampere winding number. An unbalance results in a voltage of an amplitude and a polarity which depend on the degree and polarity of said unbalance. One ring core suffices for carrying out the latter detector function, but it is necessary to involve two cores in antiphase in order to prevent the compensating circuit from suppressing the detection and in order to prevent the modulation signal from disturbing the magnetic integrator through a magnetic coupling. However, the two cores cannot be completely identical.
U.S. Pat. No. 4,536,706 discloses a magnetic current sensor for measuring the currents in a telephone circuit, i.e. relatively weak currents. This current sensor includes an offset correction circuit, and this offset correction circuit is only capable of carrying out an offset correction while the measuring of the current has been interrupted.